The field of sonic logging of boreholes in the oil and gas industry involves making acoustic measurements in the borehole at frequencies typically in the range 500 Hz-20 kHz. Below this range is typically considered as the seismic domain, above it the ultrasonic domain. A summary of the general techniques involved in borehole acoustic logging can be found in GEOPHYSICAL PROSPECTING USING SONICS AND ULTRASONICS, Wiley Encyclopedia of Electrical and Electronic Engineering 1999, pp, 340-365.
In certain well-bores, measurement of acoustic dipole signal can be difficult. This problem is nontrivial because acoustic source design is constrained by the limited space within the tool body and by the limit in power supply.
Schlumberger Technology Corporation, the assignee of this application, has provided a commercially successful acoustic logging tool, the Dipole Sonic Imaging Tool (DSI), that delays and attenuates acoustic waves propagating along the tool from the dipole source to the receiver array. The Schlumberger DSI tool attenuates acoustic waves in a manner substantially as set forth in the above-mentioned co-owned U.S. Pat. No. 5,036,945 to Hoyle et al.
The Schlumberger DSI tool is shown in schematic form in FIG. 1 (prior art). FIG. 1 (prior art) shows the DSI tool comprising a transmitter section 102 having a pair of (upper and lower) dipole sources 103 arranged orthogonally in the radial plane and a monopole source 104. A sonic isolation joint 105 connects the transmitter section 102 to a receiver section 106 which contains an array of eight spaced receiver stations, each containing two hydrophone pairs, one pair oriented in line with a lower dipole source, the other with an upper (orthogonal) dipole source. An electronics cartridge 107 is connected at the top of the receiver section 106 and allows communication between the tool and a control unit 108 located at the surface via an electric cable 109. With such a tool it is possible to make both monopole and dipole measurements. The DSI tool has several data acquisition operating modes, any of which may be combined to acquire waveforms. The modes are: upper and lower dipole modes (UDP, LDP)-waveforms, recorded from receiver pairs aligned with the respective upper and lower dipole source used to generate the signal; crossed dipole mode waveforms recorded from each receiver pair for firings of the in-line and crossed dipole source; Stoneley mode—monopole waveforms from low frequency firing of the monopole source; P and S mode (P&S) monopole waveforms from high frequency firing of the monopole source; and first motion mode—monopole threshold crossing data from high frequency firing of the monopole source.
A first advance by Schlumberger on the DSI tool increases the signal to noise ratio in a logging tool using dipole signals by shaking part of the dipole tool body axially to produce a pure, broadband acoustic dipole signal while coupling as little energy as possible into the rest of the tool body. The use of dipole signals made by shaking (axially) all or part of the dipole tool is disclosed in the above-mentioned co-pending U.S. application Ser. No. 09/537,836, filed 29 Mar. 2000. As noted above, co-owned international patent application no. PCT/IB01/00447, filed 21 Mar. 2001, claims priority to co-owned, co-pending U.S. application Ser. No. 09/537,836, filed 29 Mar. 2000, and was published 4 Oct. 2001 as international publication no. WO 01/73478 A2.
A second advance by Schlumberger on the DSI tool increases the signal to noise ratio in a logging tool using dipole signals by attaching regularly spaced mass blocks to the central mandrel within the spacer section and the receiver section of the dipole tool body. This causes the spacer section and the receiver section to behave acoustically like a mass-spring structure which does not interfere with the acoustic signals used for evaluation of the formation surrounding the borehole, while still providing suitable physical structure and support for the other parts of the tool. The use of regularly spaced mass blocks is disclosed in the above-mentioned co-owned, co-pending international patent application no. PCT/IB00/01696, filed 16 Nov. 2000, published 23 May 2002, as international publication no. WO 02/41034 A1.